I remember seeing one a few years ago for a little over a grand, but haven't seen one since. They are quite rare (I've heard speculation that there are less than 500 total examples, all of which are pre-production boards, and not all of them are functional). Expect to pay through the teeth as well; most people who have them know exactly what they have. Rampage is even rarer - there's maybe a handful of bring-up boards known to exist, which (as far as I'm aware) are nothing approaching functional as a daily-driver. They reportedly all have issues with their DAC outputs, power I/O, and the drivers are extremely rough (they're bring-up boards, after all). Very much a collectible curiosity than anything else.
As far as DVI on Voodoo4/5, the Mac editions come with DVI, and according to what I've read *can* be flashed to a PC BIOS and run under Windows or Linux. They're relatively rare in their own right though (but not unobtanium). I don't think there are Voodoo5 6000s with DVI - but I could be mistaken.
Here's an example that's on eBay right now:
http://www.ebay.com/itm/3dfx-Voodoo-5-5500-PC … =item4d1d870bb9
I don't know what the DVI output capabilities are like though; as far as I'm aware it's an "either/or" situation with the two outputs, and it probably doesn't support massively high resolutions via DVI (like many other DVI-equipped cards from that era).
Some more information about 6k and Rampage:
http://www.thedodgegarage.com/3dfx/pro_v6k.htm
http://www.thedodgegarage.com/3dfx/rampage.htm
http://www.thedodgegarage.com/3dfx/rampage_2012.htm
http://www.x86-secret.com/articles/divers/v5- … 00/v56kgb-6.htm
http://www.rashly3dfx.com/products/rampage.html
http://ixbtlabs.com/articles/3dfxtribute/index.html
As far as the 3dfx-nVidia connection, my understanding is similar to F2Bnp's in that Geforce 3 was far too new to have had 3dfx IP/engineers influence it. It takes a very long time to design new chips/boards, and the time gap between the 3dfx acquisition in late 2000/early 2001 is far too narrow for any of the 3dfx "stuff" to have made it into NV20. I've read speculation that GeForce FX reportedly implements at least some ideas that were on the drawing board for Mojo (the "next gen" after Rampage - would have likely been a GeForce 4 or later competitor). Reportedly some of the 3dfx engineering folks/ideas also wound up at ATi, according to Wikipedia. The geometry processor as an AGP interface/bridge concept explored in the above linked ixbt article isn't dissimilar from a real-world 3DLabs product, the REALiZM, which came out a few years later. I'm not sure if there's a real connection there or not though.
As far as T-Buffer, it isn't like TnL or programable shaders, it's more similar to the arcade/simulation systems that 3dfx had been building for years, in that it uses multiple GPUs (or multiple rendering pipes) to "stack" and generate effects like motion blur. You can read about it here:
http://www.beyond3d.com/content/articles/66/
And here:
http://www.beyond3d.com/content/articles/67/
Programmable shaders on modern GPUs can implement most of those effects in some manner (that is, the end result is similar enough), but it isn't the same from the hardware or API's perspective.